Electronic siren



Nov. 27, 1951 w. FLEMING 2,576,585

ELECTRONIC SIREN Filed April 19, 1948 F/aj FREQUENCY JNVENTOR. W/u/AM Haw/v6 BY 0 2 3 4 5 a 7 a 9 raw JZCONDE lUO Patented Nov. 27, 1951 ELECTRONIC SIREN William Fleming, San Leandro, Calif., assignor to Selectronics, Inc., San Leandro, Calif., a corporation of California Application April 19, 1948, Serial No. 21,779

2 Claims.

The present invention relates to audible warning devices of the type known as sirens which are adapted to generate a penetrating sound of alternately rising and falling pitch and are generally employed on police cars, fire engines, ambulances, etc.

Sirens capable of producing a signal of the kind characterized employ usually rotating vanes that are arranged to direct a current of air against a pair of concentrically superposed perforated discs one of which is rotated at great speed so that the jets of air permitted to escape when the perforations of the two discs register, are interrupted and re-established in rapid succession and thus produce a sound of a pitch determined by the speed of rotation. While the movable elements of the described structure may be driven manually, sirens employed on automotive vehicles of the type referred to are usually provided with electric motors which are energized from the batteries of said vehicles.

The operation of these motors constitutes a very heavy drain on the power stored in the batteries due to the weight of the rotary disc, which should be heavy to develop momentum, and due to the resistance of the sound producing air jets which the rotary disc must cut in continuous succession; and this drain on the power of the batteries is especially objectionable in vehicles of the type named above where the same battery may have to sustain powerful signalling lights, public address systems, radio transmitting sets and. the like.

It is an object of the present invention to furnish an electrically operated audible warning device, adapted to produce the characteristic sound of a siren, which requires a minimum of electric power to start or maintain operation.

It is another object of the present invention to furnish an audible warning device, of the type referred to, which is adapted to produce the characteristic sound of a siren without requiring manual actuation or motor power for operation.

Furthermore it is an object of the present invention to furnish an electrically operated audible warning device, of the type referred to, which involves no mechanical movement other than the vibrations of a sound producing membrane.

Additionally it is an object of the present invention to provide an arrangement that may selectively be used as a siren or as a public address system.

These and other objects of my invention will be apparent from the following description of the accompanying drawing which illustrates a preferred embodiment thereof and wherein Figure 1 is the circuit diagram of an electrically operated sound-producing device constructed in accordance with my invention,

,Figure 2 is a circuit diagram illustrating part of the system shown inFigure 1 with the symbols of its components re-arranged to facilitate an understanding of their functions, and

Figure 3 is a graph representing the performance of the arrangement illustrated in Figure 2. In accordance with my invention I provide an audio-oscillator with means automatically effective to vary the frequency of the generated oscillations and apply its varying output, with or without amplification, to a suitable loudspeaker. When such a system is energized, it produces signals that closely resemble the sound produced by the conventional acousto-mechanical sirens, but consumes far less electrical power than the motors required to operate acousto-mechanical sirens and involves no mechanical movement other than the ibrations of the loudspeaker membrane.

Having reference to the accompanying draw-- ings, the arrangement of my-invention illustrated in Figure 1 comprises an audio-oscillator 0 arranged to' feed into an audio-amplifier AA which amplifies the oscillator output and delivers it in amplified form to a loudspeaker circuit L.

As more clearly shown in 'Figure 2, the audio oscillator 0 comprises a source of D. C. voltage ID, a resistor H, a relatively large condenser l2 and a control switch l3 connected in series; and shunted across said condenser I2 is a large resistor [4 in series with a relatively small condenser |5 that is by-passed by a discharge tube It filled with an inert gas, such as neon. Tubes of this type are practically non-conductive until a sufficient voltage is placed across their electrodes to ionize the gas, said voltage being termed the firing or ionization voltage. Thereafter the tubes remain conductive until the applied voltage drops below a critical value which is generally substantially lower than the firing voltage and which is termed the extinction or deionization voltage. I

When the switch [3 is closed, both the condensers l2 and l 5 are effectively connected across the source of voltage I0, and both condensers begin to charge; and depending upon the firing characteristics of the tube Hi, the size of resistors H and I4 and the capacity of condenser l5, the voltage building up in said condenser IE will quickly reach a value adequate to ionize the causes condenser I to discharge. However, as soon as the voltage stored in said condenser drops below the extinction point of the tube I5, said tube returns to non-conductive condition permitting condenser IE to re-charge until its voltage reaches .again the firing point of ,thetube l6 whereupon another cycle of discharge followed by recharging will occur. Thus, as long as the switch I3 remains closed the condenser will continue to alternately discharge to the level set by the extinction point of tube I6 and re-charge to the level set by the firing point of said tube.

Reverting to the large condenser 1.2,;said condenser is of such size relative to the voltage applied thereto through resistor II as to require, for charging to full capacity,-a *period of time which extends over a predetermined multiple of the cyclic voltage variations occasioned in condenser !5 by the presence of the neon tube I6.-

Hence, during the initial stages in the operation of the described system .while the condenser I5 discharges and re-charges alternately in the manner explained above, the voltagein thelarge condenser I2 increasessteadily and, due to this increase in, the charge of said condenser the recharging time of condenser I5 becomesprogressively shorter with every successive cycle until the condenser I 2 ,is fully charged. At'this point then the frequency vOf the discharges of condenser I5 has reached its maximum and from there on remains constant'at said maximum as long as the switch I'd-stays closed.

As indicated, the time that will'elapse before the frequency ;of the discharges produced by the described systemreaches its maximum value depends primarily upon the charging time of the condenser I2, which in turn is'determined bythe capacity of said condenser and by the size of the timing resistor II, an increase in either being instrumental inprolonging the charging time of condenser l2. 'ITherefore, to control the-rate at which the frequency of the oscillations generated in the described manner reaches its maximum, either the condenser I2 'or the timing resistor II or both may be variable as diagrammatically indicated in Figure 2.

The frequency to which the oscillations generated by the system of my invention will rise, is determined not only by the characteristics of the neon tube I6 but also by the capacity of the relaxation condenser l5 and b the value of the voltage limiting resistor 14, a decrease in either of the latter components being instrumental in raising the upper frequency limit to higher levels; therefore, to control said upper frequency limit the resistor H may be variable as indicated in Figure 2.

Whenever the switch I3 is opened to interrupt the supply'of power, while the described arrangement is in full operation, the energy stored in the large condenser I2 will continue to charge and re-charge the relaxation condenser I5 for some time but the time required to re-charge said condenser to the firing point of its associated neon tube I6 will increase with every successive cycle due to the successive decrease of the energy stored in condenser I2, until the voltage available in the storage condenser is no longer able to charge the relaxation condenser I5 to the firing point of the tube It and the oscillations to an end. "Thus, whenever the switch I3 is opened after the described arrangement has been in operation for some time, said arrangement will produce oscillations of a decreasing frequency 4 for a period of time primarily determined by the capacity of condenser I2 relative to condenser I5, an increase in the latter or a decrease in the former being instrumental in shortening the diedown period of the oscillations. To reduce this die-down period without at the same time Shortening the period over which ethejrequency of the generated oscillations builds-up when the arrangement of my invention is set into operation,

.a bleeder resistor I8 may be shunted across the storage condenser I2, as shown in Figures 1 and :2, andto abruptly terminate the performance of the described system during the die-down period, a'normally open manually operable shunt switch I9 may be connected across the plates of the condenser I2, as shown, in order that said condenser may be instantaneously discharged,

if desired.

Reverting to Figure l the oscillations generated in the described relaxation arrangement Oare supplied through a suitable blocking condenser 20 and an isolation resistor 2I to the control grid 22 of an audio amplifier-tube -23--whichforms the first stage of the previously mentioned audioamplifier A-A. Iheanodeci-rcuit of tube 23 is coupled through an iron-cored transformer 24 to a push-pull'circuit 25 comprising -a pair of tubes 25 and 2'I respectively which have their control grids connected to opposite ends of the secondary winding of the coupling transformer 2-2 while their plates are connected to opposite ends of the primary winding of-another coupling transformer 28, the secondary Winding of which forms part of the loudspeaker circuit Lbeing connected across a suitable loudspeaker 29 as shown. 7

Thus, whenever the switch I3 of the relaxation oscillator O is closed, the electrical oscillations generated by the discharges of the condenser l5 through the neon tube "I6 are amplified in the audio amplifier AA and are supplied in appropriately amplified form to the circuit L to be converted into audible signals in the'loudspeaker 29 where they produce a penetrating sound of a rising pitch-that finallyadjusts itself to an upper frequency determined'by the constants of the relaxation arrangement 0 :as previously described; additionally, due to the filter network formed by the roupling condenser 2 0" in combination with the isolation resistor LI between the oscillator arrangement 0 and the audio amplifier AA the volume of the signal emitted by the loudspeaker will increase with increasing frequency. On the other hand whenever the switch I3 is opened, the sound emitted from the loudspeaker 29 will gradually decrease in pitch and volume andwill finally terminate unless the switch is closed again. Thus, the system of my invention is able to produce a penetrating sound of the characteristicwail of an acousto-mechanical siren, without requiring any mechanical motion other than the vibration of a loudspeaker membranemerely by-energization of electronic circuits whichirequire-far less power than the motors necessaryito operate the heavy armatures of the conventional accustomechanical sirens. In addition my invention provides a very substantial saving .in'equipment in all instances where it is desirable to have both a siren and a public address system available such as at air ports, on police cars, fire engines and the like, because a microphone circuit M may readily be connected to the amplifier AA of the device of my invention. Such an arrangement is illustrated in Figure 1 wherein an input transformer 30 delivers the voice modulations impressed by a microphone 3! upon a D. C. current supplied by a battery 32 through a suitable volume control potentiometer 33 to the control grid 22 of the previously described audio amplifier tube 23. Hence any address or Warning spoken into the microphone 3i is amplified in the audio amplifier AA and delivered to the public in properly amplified form through the same loudspeaker 29 that emits the siren signals whenever the switch [3 of the relaxation oscillator O is manipulated.

As an added advantage my invention permits pre-adjustment of sirens of identical construction to signals of distinctly different character depending upon the purpose for which they may be used. By pre-setting. the timing resistor ll and/or the storage condenser l2, it is possible to shorten or extend the build-up period of the signal at will, and by adjusting the voltage-limiting resistor [4 the frequency to which the signal will rise may be predetermined to give it a distinctly high or low characteristic; furthermore by adjustment of the bleeder resistor 18 the diedown period of the signal may be shortened or extended so that it will end abruptly or roll on for some time. Thus, by combining selected ones of these alternatives the sirens of my invention may be given an one of a wide range of distinct sound characteristics that may identify their place of origin as an ambulance, a police car, a fire engine, or whatever other purpose they may serve.

In a practical embodiment of my invention constructed according to the diagram shown in Figure 2, I employed components having the following constants:

Resistor l I, 200,000 ohms. Resistor I4, megohms. Resistor l8, 5 megohms. Resistor 2 l, 500,000 ohms. Condenser I2, 4 microfarads. Condenser I5, 500 micromicrofarads. Condenser 20, 500 micromicrofarads. Neon tube I6:

Ionization voltage, '75 volts. Deionization voltage, 65 volts. Source of power [0, 250 volts plus.

Figure 3 illustrates the rate of increase in the frequency of the generated oscillations obtained in a device employing the above stated circuit constants and indicates that the system commenced to oscillate in less than 1 second after the switch [3 was closed, but required more than four seconds thereafter before the frequency climbed to its constant maximum value of 600 cycles per second. Comparative tests showed that it required less than 50 watts to set the system of my invention into operation whereas more than 1200 watts are frequently required to start operation of conventional acousto-mechanical sirens.

While I have explained my invention with the aid of a specific embodiment thereof, I do not wish to be limited to the particular circuit arrangement shown in the drawing of the exemplary data stated in the specification, both of which may be departed from without departing from the scope and spirit of my invention. Thus, although I have described and illustrated a particular audio amplifier, it will be understood that other audio amplifiers may be successfully employed in the system of my invention. Also, while I have disclosed a particular audio oscillator as forming an essential part of my novel siren, the man skilled in the art, once acquainted with the principles of my invention, will be able to adapt other oscillators for operation within the system of my invention by arranging them in accordance with my invention in such a manner rat the frequency of their oscillations will vary automatically in a predetermined pattern.

Having thus described my invention and the manner in which it is to be performed, what I claim is:

1. An electronic sires including in combination an audio oscillator comprising a first resistor, a first capacitor and a control switch in series, a second resistor and a second capacitor in series connected across said first capacitor, said second capacitor being of a smaller value than said first capacitor, a gas discharge tube shunted directly across said second capacitor and a bleeder resistor and a shunt switch each connected individually across said first capacitor; an audio amplifier arranged to amplify the oscillations generated by said oscillator, and a loudspeaker arranged to reproduce the output of said amplifier.

2. An electronic siren including in combination an audio oscillator comprising a first resistor, a first capacitor and a control switch in series, a second resistor and a second capacitor in series connected across said first capacitor, said second capacitor being of a smaller value than said first capacitor, and a gas discharge tube shunted directly across said second capacitor; an audio amplifier arranged to amplify the signals generated by said oscillator; a loud speaker arranged to reproduce the output of said amplifier; and a normally open shunt switch connected directly across said first capacitor and adapted upon closing to instantaneously terminate a generated signal.

WILLIAM FLEMING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,994,902 'Irouant Mar. 19, 1945 2,121,434 Klinedinst et al. June 21, 1938 2,140,840 Langer Dec. 20, 1938 2,235,667 Blount Mar. 18, 1941 2,353,499 Purington July 11, 1944 2,354,699 Owens Aug. 1, 1944 2,439,844 Davis Apr. 20, 1948 FOREIGN PATENTS Number Country Date 561,304 Great Britain May 14, 1944 

